1. Field of the Invention
This invention generally relates to compositions and methods or processes for gravel packing and sand control in wells drilled in subterranean hydrocarbon-bearing formations, including without limitation horizontal and directional wells. This invention is particularly advantageous for use in water sensitive formations.
2. Description of Relevant Art
As used herein, the terms “gravel pack fluid,” “gravel packing fluid,” or “carrier fluid,” which may be used interchangeably herein, shall be understood to mean a fluid used for sand control, gravel packing, and workover operations, as well as a fluid employed as a packer fluid. The term “gravel” shall be understood to include not only natural gravel but other proppant type materials, natural and man made or synthetic, such as, for example, sand, pebbles, and synthetic beads. Generally, the well's bottom hole pressure, which must be equalized, determines the minimal density of the fluid needed. The term “solids-free” as applied to the basic gravel pack fluid shall be understood to mean that no solid materials (e.g., weighting agents or commercial particulates) are present in the fluid (except that the term is not intended to exclude the presence of drill cuttings in the fluid in the well). The term “horizontal” with respect to a wellbore or to drilling shall be understood to mean at an angle or incline other than 90 degrees from the wellbore surface as the surface is viewed as a horizontal plane at ground level. The term “brine-sensitive formation” shall be understood to mean a formation sensitive to brines or that is best suited for oil-based drilling and packer fluids, such as a formation having swelling or sloughing shales, salt, gypsum, anhydrite or other evaporite formation, a hydrogen-sulfide containing formation and hot (greater than 300 degrees Fahrenheit) holes.
During the drilling of a wellbore, various fluids are typically used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore, and then may subsequently flow upward through the wellbore to the surface. Common uses for well fluids include: lubrication and cooling of drill bit cutting surfaces while drilling generally or drilling-in (i.e., drilling in a targeted payzone or petroliferous formation), transportation of “cuttings” (pieces of formation dislodged by the cutting action of the teeth on a drill bit) to the surface, controlling formation fluid pressure to prevent blowouts, maintaining well stability, suspending solids in the well, minimizing fluid loss into and stabilizing the formation through which the well is being drilled, fracturing the formation in the vicinity of the well, displacing the fluid within the well with another fluid, cleaning the well, testing the well, transmitting hydraulic horsepower to the drill bit, emplacing a packer, abandoning the well or preparing the well for abandonment, and otherwise treating the well or the formation.
Once the well has been drilled and a hydrocarbon reservoir has been encountered, the well is ready to be completed. In the course of completing a well, it is common practice to run a string of casing into the well bore and then to run the production tubing inside the casing. At the producing interval(s) of the formation, perforations are typically created to extend through the casing string, through the cement that secures the casing string in place, and a short distance into the formation. These perforations may be created by detonating shaped charges carried in a perforating gun. The perforations created cross one or more production zones to allow production fluids to enter the interior of the wellbore.
After the well is cased and perforated, a stimulation or sand control treatment process may be performed. Sand control processes may prevent, after the well is completed and placed in production, formation sand from unconsolidated formations being swept into the flow path along with formation fluid, which erodes production components in the flow path. Similarly, in uncased boreholes, where an open face is established across the oil or gas bearing zone, formation sand from unconsolidated formations may also be swept into the flow path along with formation fluid.
Thus, with either cased or uncased well bores, one or more sand screens may be installed in the flow path between the production tubing and the rock face in the producing reservoir. Additionally, the annulus around the screen may be packed with a relatively coarse sand or gravel into the void between the reservoir rock and the outside of the screen, so as to act as a filter to reduce the amount of line formation sand reaching the screen, to support the porous medium of the producing reservoir so that it will not collapse into the void between the reservoir rock and the outside of the screen, and to seal off the annulus in the producing zone from non-producing formations. When the sand tries to move through the gravel, it is filtered and held back by the gravel and/or screen, but formation fluids continue to flow unhindered (by either the gravel or screen) into the production string.
In deep wells, reliability of the sand face completion is very important, due to the prohibitively high cost of intervention. Further, as many such wells are completed open hole and in relatively incompetent rock, gravel packing of open-hole horizontal wells is increasingly becoming a standard practice in the deep-water, sub-sea completion environment. The gravel packing process involves mixing gravel with a carrier fluid, and pumping the slurry down the tubing and through the cross-over, thereby flowing into the annulus between the screen and the wellbore. The carrier fluid in the slurry leaks off into the formation and/or through the screen. The screen is designed to prevent the gravel in the slurry from flowing through it and entering the production tubing. As a result, the gravel is deposited in the annulus around the screen where it becomes tightly packed, forming a “gravel pack.” Thus, gravel is deposited adjacent an open hole where it serves to prevent sand and other formation fines from flowing into the wellbore.
Proper selection of the carrier fluid is essential to a gravel packing process. Ideally, the carrier fluid shall not cause any permeability reduction of the formation. When viscous fluids are used, carrier fluid must also have sufficient viscosity to suspend and carry the gravel during placement. Carrier fluids are either considered “water-based” or “oil-based” depending on the constituency of their external continuous phase. Aqueous-based fluids can be tailored to be compatible with many formations simply by including salts such as, for example, potassium chloride, ammonium chloride, or tetramethyl ammonium chloride. Consequently, to date, the convention in gravel-packing horizontal wells has been water packing or shunt-packing with water-based viscous fluids comprising a brine, a gelling agent such as, for example, hydroxyethylcellulose (HEC), xanthan or a viscoelastic surfactant, and breakers to minimize the pressure required to move the fluid back to the wellbore.
However, when a well has been drilled with an oil or synthetic-based drilling fluid, aqueous based fluids can be incompatible with the drilling fluid or with the subterranean formation having characteristics that necessitated the use of the oil or synthetic-based drilling fluid such as a brine—sensitive formation. Clean hydrocarbon oils would likely provide the most compatible or least damaging completion fluids, but such oils do not have the required densities and do not readily dissolve compounds that could provide the required densities. Invert emulsion based gravel pack fluids have been used with a typical oil:water ratio of 30:70 (i.e., the oil content being less than the brine content). While these fluids are sufficiently viscous to carry the gravel, they have the disadvantage of necessitating the breaking of the emulsion to lower the viscosity of the fluid during flow-back, that is, to lower the viscosity so the fluid can return to the surface after depositing the gravel or other gravel packing material.
Accordingly, there exists a continuing need for developments in carrier fluids for gravel packing processes.